This invention relates to a method and apparatus for melting and refining metals, particularly steel. More particularly, it relates to a lance/burner means for preheating a quantity of metal, usually scrap, with an oxy-fuel flame or a fuel/air/oxygen flame, and also for refining such metal by directing a high pressure oxygen stream toward the melt.
A known method of metallurgical processing, i.e. steel production, involves the injection of oxygen into the melt of charged materials, such as pig iron and scrap, for refining purposes. Presently, oxygen injection is accomplished by three distinct oxygen-lancing techniques.
One technique utilizes consummable lance pipes submerged at one end into the melt, and connected to an oxygen supply at the other end. A second technique, which utilizes a watercooled lance, involves a length of pipe surrounded by a watercooled jacket which is introduced into the furnace prior to the refining cycle and positioned to inject a high pressure oxygen stream into the melt by use of a specially designed nozzle. The third technique involves a watercooled oxygen lance/burner combination. In this method, a watercooled movable lance/burner produces a flame to melt scrap at the front of its nozzle during melting and provides a high-pressure oxygen stream from the same nozzle toward the molten bath during refining.
In the latter two watercooled lance and lance/burner techniques, the oxygen stream is directed toward the melt rather than submerged into the molten bath. Many oxygen lances use a Laval nozzle to provide the capability of penetrating the melt with oxygen at supersonic velocity, thereby increasing the interaction of oxygen and molten metal. Positioning the end of the lance at a distance of approximately one to two feet from the melt increases the penetration of the oxygen stream below the surface of the melt.
An advantage of the use of a watercooled lance/burner combination, rather than just a lance, is that it is capable of providing additional energy input by combustion of auxiliary fuel during melting, which is not possible with either the submerged pipe or the watercooled lance technique.
A disadvantage of using the watercooled lances and lance/burners described above is that it is necessary to completely remove them from the furnace and furnace shell after the lancing operation has been completed. This results in the need for keeping the lance-insertion opening in the furnace wall clean of slag and steel. For this reason the opening must be located a relatively long distance away from the top level of the molten metal. This in turn dictates that the lance pipe be of long length and also that a heavy structure be provided to support and move the lance in and out of the furnace. Also, the long length of the lance necessitates the increase of the diameter and surface area of the watercooled jacket, which in turn results in a corresponding excessive loss of heat and the consumption of cooling water. The long distance that the lance must travel through the furnace interior requires that the lance be positioned for insertion at the last possible moment when the scrap is almost completely melted and when the scrap pile will not interfere with the movement of the lance. This delays the refining cycle, thereby slowing down the steel-making process.
Lance/burner combinations in particular have severe maintenance problems during lancing due to plugging of the combustion tunnel and the fuel orifices of the lance/burner. The flow of fuel is shut down or reduced and the lance/burner nozzle is positioned close to the molten bath. Severe splashing of molten slag and metal during oxygen injection results from a rapid oxidation reaction between oxygen and molten metal components including, but not limited to, silicon, manganese, cromium, carbon and iron. The combustion tunnel and the fuel orifices experience plugging due to the splattering of steel and slag during refining which leads to the deterioration of the efficiency of the device as a burner.
There exists, therefore, a need for a lance which injects a high pressure oxygen stream into a melt and which permits the delivery of heat from the combustion of auxiliary fuel into scrap prior to refining. Also, there exists a need for such a lance which permits the early introduction of oxygen into the pool of melted material for refining purposes. A further need is for a lance of relatively small size so that its surface area and resultant loss of heat to the furnace, as well as its required cooling water pressure and consumption, is minimized. A still further need is for a lance which overcomes the problem of splashing steel and slag plugging its combustion tunnel and fuel orifices.